In essence, says Gillian Foulger of Britain's Durham University, the scientists were watching a movie of the magma moving into the volcano and up to the surface.

"They only have two snapshots," she said, "but we're moving toward a situation where we can have many snapshots that show us the progress."

In an accompanying comment in Science, Foulger calls the Italians' work an important step toward the technology presented in the 2005 Discovery Channel movie Supervolcano. In the film, scientists of the future could track magma movements and issue warnings to people likely to be in harm's way.

Study co-author Patanè agrees. "This discovery will potentially offer a powerful tool of integrated surveillance for forecasting eruptions in the middle and short term, in particular those of highly explosive nature," he said.

Powerful Technique

This type of intensive seismological mapping can also be used to study other types of subterranean features.

Foulger, for example, has used a similar method to track the effect of geothermal power generation on underground steam reservoirs at The Geysers hydrothermal site in northern California.

The results indicated that excessive power generation was depleting the underground steam, leading to changes in how the power plants are operated.

In another study, her team made a detailed seismic survey of California's Mammoth Mountain, a popular ski area that sits in a zone of restless volcanism.

At the time, there had been many small earthquakes, "suggesting that we might be building up for an eruption sometime in the future," she said.

"There was no eruption, but carbon dioxide started pouring out of the mountain at an enormous rate, killing off the forests."

She and her colleagues conducted an eight-year study there, observing how carbon dioxide gas appeared to have accumulated in some places while moving out of others.

This is useful, she says, because the movement of gases and water affects how explosive an eruption will be if one occurs.

Foulger dreams of a future in which all potentially dangerous volcanoes will be outfitted with networks of seismometers similar to the ones that allowed Patanè's research group to watch lava movements beneath Etna.

"We have the technologies, but there is often a huge reluctance to implement them until too late," she said.

"The Italian paper is very helpful, because it's a step forward in showing what is possible. Politicians and people in communities that are potentially in danger from active volcanoes should take note," she added.

"Maybe we can consider implementing it before an eruption rather than after."